As stated in the release notes for versions 1.9.0.1 and 1.8.3, the addon itself continues to function in Firefox 8 and above with the exception that the browser now treats the application/json MIME type as text. I was planning to make a new release of JSONovich (tomorrow if all goes well) addressing the issue and several others in time for Firefox 8's move from beta to the release channel on November 8th. If this functionality had been essential, a fix was committed to source control at http://code.google.com/p/jsonovich/source/detail?r=151 over a month ago.

Apologies that your experience was less than optimal. However with fewer than 2.5% of JSONovich's active users on Firefox's beta channel (that's less than half the number of users still on Firefox 3.6) I had not prioritised a release, especially considering review times of over a week - I didn't consider it important enough alone to warrant adding to editor burden.

EDIT: JSONovich 1.9.2 is now available (from https://addons.mozilla.org/addon/jsonovich/versions/ until review is complete) that contains a workaround for this minor conflict with a new Firefox 8 feature.